1. Field of the Invention
This invention relates generally to a surface acoustic wave (SAW) reflector filter and, more particularly, to a SAW reflector filter employing dual tracks each including an input transducer, an output transducer and at least one reflector, where the reflectors have reflection functions that are equal in magnitude and opposite in phase so that the reflected waves combine at the output transducers.
2. Discussion of the Related Art
Surface acoustic wave (SAW) filters for use in mobile phone communications systems are designed to be small in size, exhibit good out-of-bandwidth rejection, and provide narrow bandwidths with steep transition edges. Conventional SAW filters include an input transducer and an output transducer formed on a piezoelectric substrate. The input transducer is electrically excited with the electrical input signal that is to be filtered. The input transducer converts the electrical input signal to surface acoustic waves, such as Rayleigh waves, lamb waves, etc., that propagate along the substrate to the output transducer. The output transducer converts the acoustic waves to a filtered electrical signal.
The input and output transducers typically include interdigital electrodes formed on the top surface of the substrate. The shape and spacing of the electrodes determine the center frequency and the band shape of the acoustic waves produced by the input transducer. Generally, the smaller the width of the electrodes, or the number of electrodes per wavelength, the higher the operating frequency. The amplitude of the surface acoustic waves at a particular frequency is determined by the constructive interference of the acoustic waves generated by the transducers.
The combined length of the transducers determines the length of the overall filter. To design a conventional SAW filter with ideal filter characteristics, the filter""s impulse response needs to be very long. Because the length of the impulse response is directly proportional to the length of the transducer, the overall length of a conventional SAW filter having ideal characteristics would be too long to be useful in mobile phone communication systems.
Reflective SAW filters have been developed to satisfy this problem. Reflective SAW filters generally have at least one input transducer, one output transducer and one reflector formed on a piezoelectric substrate. The reflector is typically a reflective grating including spaced apart grid lines defining gaps therebetween. The acoustic waves received by the reflector from the input transducer are reflected by the grid lines within the grating so that the reflected waves constructively and destructively interfere with each other and the wave path is folded. The constructively interfered waves are reflected back to the output transducer having a particular phase. Because of the folding, the length of the transducer is no longer dependent on the duration of the impulse response. Reflective SAW filters are, therefore, smaller in size and have high frequency selectivity, and thus are desirable for mobile phone communications systems.
The frequency response of a reflective SAW filter is further improved by weighting the individual reflectors to achieve a desired net reflectivity. The frequency response sets the phase and magnitude of the reflected acoustic waves. Existing weighting methods include position-weighting, omission-weighting and strip-width weighting. Other methods of weighting reflectors include changing the lengths of open-circuited reflective strips within an open-short reflector structure. Weighting the reflector helps to reduce the physical size of the filter and to improve the filter""s frequency response.
FIG. 1 is a top plan view of a known dual track SAW reflector filter 10 including a first track 12 and a second track 14. The first track 12 includes a bi-directional input interdigital transducer 16, a bi-directional output interdigital transducer 18, a first reflector 20 positioned on one side of the input transducer 16 and a second reflector 22 positioned on an opposite side of the output transducer 18, all formed on a piezoelectric substrate 24, as shown. Likewise, the second track 14 includes a bi-directional input interdigital transducer 28, a bi-directional output interdigital transducer 30, a first reflector 32 positioned on one side of the input transducer 28 and a second reflector 34 positioned on an opposite side of the output transducer 30, all formed on the piezoelectric substrate 24, as shown. The reflectors 20, 22, 32 and 34 can be any one of a number of suitable reflector devices, such as a reflective grating including a series of grid lines. The interdigital transducers 16, 18, 28 and 30 include a plurality of uniformly spaced interdigital electrode fingers 38 attached at opposite ends by bus bars 40.
An electrical input signal to be filtered is applied to the input transducers 16 and 28 on an input line 42. The input transducers 16 and 28 convert the electric signal into surface acoustic waves that propagate outward from the input transducers 16 and 28 along a top surface of the substrate 24. Some of the acoustic waves from the input transducer 16 are directed towards the reflector 20 and some of the acoustic waves from the input transducer 16 are directed towards the output transducer 18 and the reflector 22. Likewise, some of the acoustic waves from the input transducer 28 are directed towards the reflector 32 and some of the acoustic waves from the input transducer 28 are directed towards the output transducer 30 and the reflector 34.
The reflectors 20, 22, 32 and 34 are tuned to the wavelength xcex at the center frequency of the frequency band of interest that is to be filtered, and have the same length L1. The reflected waves from the reflectors 20 and 22 are directed back to the output transducer 18 and the reflected waves from the reflectors 32 and 34 are directed back to the output transducer 30 where they are converted to a filtered electrical signal on a common output line 36.
The input transducer 16 and the output transducer 18 are spaced the same distance apart (L3) as the input transducer 28 and the output transducer 30. Also, the output transducers 18 and 30 have opposite polarities. Therefore, the surface acoustic waves directly received by the output transducer 18 from the input transducer 16 are 180xc2x0 out of phase with the surface acoustic waves directly received by the output transducer 30 from the input transducer 28. Hence, these waves cancel on the output line 36 and will not be converted into electrical signal at the output transducers 18 and 30. These waves pass through the output transducers 18 and 30 with little attenuation and are reflected by the reflector 22 in the track 12 and the reflector 34 in the track 14, respectively.
It is necessary to prevent cancellation of the reflected acoustic waves from the reflectors 20, 22, 32 and 34 on the output line 36. The reflectors 20 and 32 and the reflectors 22 and 34 are thus offset relative to each other by xcex/4. Particularly, the distance between the input transducer 16 and the reflector 20 and the distance between the output transducer 18 and the reflector 22 is L2. However, the distance between the input transducer 28 and the reflector 32 and the distance between the output transducer 30 and the reflector 34 is L2+xcex/4. Thus, the acoustic waves reflected by the reflectors 20 and 32 travel a different distance to the transducers 18 and 30, respectively, by xcex/2, and are thus 180xc2x0 out of phase with each other when they reach the output transducers 18 and 30. In other words, the acoustic waves in the second track 14 are delayed relative to the acoustic waves in the first track 12. Therefore, the output signals add on the output line 36. Likewise, the acoustic waves reflected by the reflectors 22 and 34 travel a different distance to the output transducers 18 and 30, respectively, by xcex/2, and thus are out of phase with each other when they reach the output transducers 18 and 30. Therefore, these signals also add on the output line 36. Hence, only reflected acoustic waves are provided on the output line 36.
FIG. 2 is a top plan view of a known SAW filter 50, similar to the filter 10 discussed above, including a first track 52 and a second track 54. In this embodiment, each of the transducers and reflectors are tapered to accommodate a series of contiguous communications channels. Particularly, the first track 52 includes a tapered bi-directional input interdigital transducer 56, a tapered bi-directional interdigital output transducer 58, a first tapered reflector 60 adjacent to the input transducer 56 and a second tapered reflector 62 adjacent to the output transducer 58, all formed on a piezoelectric substrate 64. Likewise, the second track 54 includes a tapered bi-directional input interdigital transducer 68, a tapered bi-directional output interdigital transducer 70, a first tapered reflector 72 adjacent to the input transducer 68 and a second tapered reflector 74 adjacent to the output transducer 70 and opposite to the input transducer 68, all formed on the substrate 64. The reflectors 60, 62, 72 and 74 and the transducers 56, 58, 68 and 70 are tapered, or have varying grating and finger widths, so that the filter 50 becomes a relatively wide fractional bandwidth filter, as is well understood in the art.
The input transducers 56 and 68 are coupled to a common input line 76 and the output transducers 58 and 70 are coupled to a common output line 78. As above, the reflectors 60 and 72 and the reflectors 62 and 74 are offset relative to each other by xcex/4 so that the reflected wave signals are out of phase with each other when they reach the output transducers 58 and 70, and thus add on the output line 78. Also, as above, the direct waves from the input transducer 56 to the output transducer 58 at the first track 52 cancel the direct wave from the input transducer 68 to the output transducer 70 at the second track 54.
FIG. 3 is a top plan view of another known dual track SAW reflector filter 90, and is particularly disclosed in U.S. Pat. No. 5,661,444 issued Aug. 26, 1997 to Dill et al. The SAW filter 90 includes a first track 92 and a second track 94. The first track 92 includes an input single phase unidirectional transducer (SPUDT) 96, an output SPUDT 98 and a reflector 100 positioned between the SPUDTs 96 and 98, all formed on a piezoelectric substrate 102, as shown. Likewise, the second track 94 includes an input SPUDT 106, an output SPUDT 108 and a reflector 110 positioned between the SPUDTs 106 and 108, all formed on the substrate 102, as shown. The input SPUDTs 96 and 106 are electrically coupled to a common input line 112 and the output SPUDTs 98 and 108 are electrically coupled to a common output line 114.
The electrical signal to be filtered is applied to the input line 112 and causes the SPUDT 96 to generate unidirectional surface acoustic waves that propagate along the substrate 102 towards the reflector 100, where they are reflected within the grating structure of the reflector 100 to provide the longer impulse response and phase control. Likewise, the electrical signal on the line 112 applied to the SPUDT 106 generates unidirectional surface acoustic waves that propagate along the piezoelectric substrate 102 towards the reflector 110 to be reflected therein. Reflections within the grating structure of the reflectors 100 and 110 provide signal cancellation and propagation to provide a transmission wave that is phase controlled. The surface acoustic waves that are phase controlled by the reflector 100 are received by the output SPUDT 98 and the surface acoustic waves that are phase controlled by the reflector 110 are received by the output SPUDT 108.
Because the output SPUDTs 98 and 108 have opposite polarities, if the distance between the SPUDT 96 and the SPUDT 98 was the same as the distance between the SPUDT 106 and the SPUDT 108, and the reflectors 100 and 110 were the same length and had the same phase properties, the signals would cancel on the output line 114. To prevent this signal cancellation, the ""444 patent proposes making the reflectors 100 and 110 different lengths so that the acoustic waves in the track 94 are delayed relative to the surface acoustic waves in the track 92 so that they add at the output line 114. It is noted that the reflector 100 is centered between the SPUDTs 96 and 98 and the reflector 110 is centered between the SPUDTs 106 and 108.
In this example, the reflector 110 has a length L3 and the reflector 100 has a length L3xe2x88x92xcex/2. Therefore, the surface acoustic waves from the SPUDT 106 received by the SPUDT 108 are delayed by xcex/2 relative to the surface acoustic waves generated by the SPUDT 96 and received by the SPUDT 98. Because of this delay, the surface acoustic waves received by the SPUDTs 98 and 108 are out of phase with each other when they reach the output SPUDTs 98 and 108, respectively, and thus add on the output line 114.
There are two fundamental problems with the SAW reflector filter 90. First, for optimum performance, the reflector grating reflectivity of the reflectors 100 and 110 can only reflect 50% of the SAW energy from the SPUDTs 96 and 106, and thus only 50% of the useful energy is transmitted from the SPUDTs 96 and 106 through the reflectors 100 and 110 to the output SPUDTs 98 and 108, respectively. The ideal insertion loss of the filter 90 is 6 dB. If other secondary effects, such as propagation loss, resistive loss, diffraction loss, matching circuit loss, etc., are included, the insertion loss of a realistic device will be about 10 dB or more. Secondly, the configuration of the filter 90 provides multiple spurious responses in the time domain after the main signal due to the multiple reflections of the acoustic waves between the SPUDTs 96 and 106 and the reflector gratings 100 and 110. These multiple spurious signals are undesirable because they cause large passband ripples and group delay ripples in the frequency domain. These multiple reflections are most prominent when the insertion loss is matched to the lowest level by the external matching circuits.
FIG. 4 is a top plan view of a known dual track SAW reflector filter 120 of the type disclosed in U.S. Pat. No. 5,896,072 issued Apr. 20, 1999 to Bergman et al. The SAW filter 120 includes a first track 122 and a second track 124. The first track 122 includes a bi-directional input interdigital transducer 126, a reflector 128 positioned on one side of the transducer 126 and an output SPUDT 130 positioned on an opposite side of the transducer 126, all formed on a piezoelectric substrate 132. Likewise, the second track 124 includes a bi-directional input interdigital transducer 134, a reflector 136 positioned on one side of the transducer 134 and an output SPUDT 138 positioned on an opposite side of the transducer 134, all formed on the substrate 132, as shown. The transducers 126 and 134 are coupled to a common input line 140 and the SPUDTs 130 and 138 are coupled to a common output line 142. The transducers 126 and 134 have the same polarity and the SPUDTs 130 and 138 have opposite polarities.
In this embodiment, the input transducer 126 and the output SPUDT 130 are the same distance apart (L4) as the input transducer 134 and the output SPUDT 138. Therefore, the acoustic waves received by the SPUDTs 130 and 138 directly from the input transducers 126 and 134, respectfully, are in phase, and thus cancel on the output line 142 because the SPUDTs 130 and 138 have opposite polarities. However, the distance between the input transducer 126 and the reflector 128 is L2 and the distance between the input transducer 134 and the reflector 136 is L2+xcex/4. Therefore, the surface acoustic waves reflected by the reflectors 128 and 136 are out of phase with each other when they reach the SPUDTs 130 and 138, respectively, and thus add on the output line 142 because the SPUDTs 130 and 138 have opposite polarities.
There is one fundamental problem with the reflector filter 120 that limits its performance. In order to design a low loss filter using this configuration, the SPUDTs 130 and 138 must be strong. However, strong reflections from the SPUDTs 130 and 138 also results in a strong spurious response in the time domain after the main signal due to the multiple reflections between the SPUDTs 130 and 138 and the reflectors 128 and 136, respectively. This spurious response is undesirable because it causes large passband ripples and group delay ripples in the frequency domain. The spurious response is most prominent when the insertion loss is matched to the lowest level by the external matching circuits.
In accordance with the teachings of the present invention, a dual track SAW reflector filter is disclosed. The filter includes a first track having a first input transducer, a first output transducer, a first reflector and a second reflector. The filter further includes a second track having a second input transducer, a second output transducer, a third reflector and a fourth reflector. The distance between the input transducer and the output transducer in the first track is the same as the distance between the input transducer and the output transducer in the second track. The distance between the first input transducer and the first reflector in the first track, the distance between the first output transducer and the second reflector in the first track, the distance between the second input transducer and the third reflector in the second track and the distance between the second output transducer and the fourth reflector in the second track is the same. Also, the length of all the reflectors is the same. In an alternate embodiment, the first track and the second track may include only one reflector.
The reflectivity function of the first reflector in the first track and the third reflector in the second track are equal in magnitude and opposite in phase. The reflectivity function of the second reflector in the first track and the fourth reflector in the second track are also equal in magnitude and opposite in phase. The input transducers have the same polarity and the output transducers have opposite polarities.
Surface acoustic waves produced by the input transducers and received directly by the output transducers are in phase with each other when they reach the output transducers, and thus cancel on a common output line electrically coupled to both of the output transducers. The acoustic waves then pass through the output transducers with little attenuation and reach the third reflector in the first track and the fourth reflector in the second track. Surface acoustic waves produced by the first input transducer, reflected by the first reflector and received by the first output transducer in the first track are 180xc2x0 out of phase with surface acoustic waves produced by the second input transducer, reflected by the third reflector and received by the second output transducer in the second track, and thus combine on the common output line. Surface acoustic waves produced by the first input transducer, reflected by the second reflector and received by the first output transducer in the first track are 180xc2x0 out of phase with surface acoustic waves produced by the second input transducer, reflected by the fourth reflector and received by the second output transducer in the second track, and thus also combine on the common output line.
Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.